Myostatin (MSTN) and GDF-11 are secreted proteins that we originally identified in a screen for novel growth and differentiation factors related to transforming growth factor-I_ (TGF-13). The predicted sequences of MSTN and GDF-11 are greater than 90% identical in the mature, C-terminal portion of the proteins, and together, these molecules form their own subgroup within the larger TGF-13 superfamily. We have been using a variety of in vitro and in vivo approaches, including gene targeting in mice, to attempt to identify the biological functions of MSTN and GDF-11. We have shown that mice lacking MSTN have dramatic and widespread increases in skeletal muscle mass, suggesting that MSTN normally functions as a negative regulator of muscle growth. We have also shown that mice lacking GDF-11 have extensive homeotic transformations of the axial skeleton, suggesting that GDF-11 normally acts as a global regulator of axial patterning. The overall aim of this proposal is to further investigate the biological functions of these molecules and the mechanisms by which their activities are regulated. The specific aims are: to investigate the functional redundancy of MSTN and GDF-11; to analyze the effect of postnatal loss of MSTN and GDF-11 on skeletal muscle mass; to further characterize the role of activin type II receptors in regulating MSTN and GDF-11 signaling; to identify other components of the MSTN and GDF-11 receptor complex; to further investigate the role of follistatin in regulating MSTN and GDF-11 activity; and to investigate the mechanism by which latent MSTN is activated. Taken together, these studies will provide important insights into the normal biological functions of these molecules and may suggest new strategies for modulating the activities of these molecules for human therapeutic applications in muscle wasting diseases, such as muscular dystrophy and cachexia, and metabolic diseases, such as obesity and type II diabetes.